The invention relates to brazing filler metals and, in particular, to brazing filler metals for use in preparing composite materials, including metal composites. In a particular embodiment, the invention relates to a brazing filler metal containing silver, copper, nickel and, optionally, silicon. The brazing filler metals of the invention may be in the form of foil, wires, powders and pastes. The present invention also relates to composite materials made using the brazing filler metals, and to composite articles fabricated therefrom, including steel bearings or bushings, and to methods of preparing the composite materials and articles.
Brazing is a method similar to welding for use in joining two or more materials, including metals, ceramics, or combinations thereof. Heat is applied to a brazing filler metal, which liquefies and flows by capillary action into the gap separating the materials. The brazing filler metal solidifies to create a bond at the molecular level thereby joining the materials.
Brazing can be used to join metals and ceramics in structural applications. The capillary flow of molten brazing filler metals can also be used to infiltrate or impregnate porous materials, including metals and ceramics. As a result, brazing technology can be used to prepare composite materials by providing a molten brazing filler metal to fill pores in matrix materials.
A variety of brazing filler metals are known in the art, including brazing filler metals based on precious and non-precious metals. Variables to be considered in selecting a particular brazing filler metal include cost, operating temperature of the brazement and wettability of the material to be joined by the brazing material. xe2x80x9cWettabilityxe2x80x9d describes the ability of a solid surface to accept the flow and adherence of a liquid (i.e., the molten brazing filler metal). In general, a brazing filler metal typically wets at or near the liquidus temperature of the active metal component, typically greater than 450xc2x0 C.
Brazing filler metals based on silver are commonly used to join metal or ceramic materials, as well as to infiltrate and coat porous metals or alloys. For example, brazing filler metals comprising silver, copper, and optionally zinc, are commonly used to join copper, steel, Kovar and nickel alloy components in a variety of industrial applications. American Welding Society, Brazing Handbook (4th ed. 1991).
Silver-based brazing filler metals are also used to infiltrate and coat powder metallurgy components having less than 93% theoretical density, including steel components. Steel powder metallurgy components can be fabricated to form a matrix having an interconnected porosity. The infiltration brazing filler metal is typically formed into a wire, wrapped around a flux coated component, and heated. The brazing filler metal infiltrates, coats and fills these pores, significantly increasing the density and hardness of the matrix component. Infiltrated matrix components, termed composite materials, are then fabricated into composite articles such as bearings or bushings. The improved surface of the infiltrated matrix component reduces wear on the bearing surface. Silver""s high lubricity and resistance to galling make this metal particularly desirable for use in fabricating bearings, bushings, and similar load bearing parts.
An 85% silverxe2x88x9215% manganese alloy, in the form of coiled rings, is used to infiltrate and coat steel powder metallurgy components. As with many silverxe2x88x92based brazing filler metals, the 85% silverxe2x88x9215% manganese alloy requires the presence of a flux during furnace brazing in hydrogen environments. The flux forms a protective coating which prevents oxidation, removes oxides, and reduces fuming during the brazing process. Brazing flux may be made of fluoride, chloride, borax, boric acid or borates. Thus, when an 85% silverxe2x88x9215% manganese alloy is used to infiltrate and coat steel powder metallurgy components, a chloride flux is typically used to prevent formation of manganese oxides. Fluxing is undesirable because it adds to the cost and duration of the brazing process. Moreover, the infiltration of the flux itself into the pores can block braze infiltration, leading to a decrease in density, hardness and toughness of the metal composite.
The use of the 85% silverxe2x88x9215% manganese alloy presents several other disadvantages. First, the 85% silverxe2x88x9215% manganese alloy can only be hardened by strain-hardening by cold work. This results in an increase in hardness and strength, but a decrease in ductility. Moreover, increasing the strength of the brazement by cold working is both costly and infeasible. Second, the 85% silverxe2x88x9215% manganese alloy is both difficult and costly to manufacture. Specifically, alloys containing manganese tend to react with graphite molds. As a result, the ingots of these alloys can be difficult to remove and the lifetime of the mold is consequently reduced. Furthermore, alloys containing manganese must be treated chemically (i.e., by acid pickling) before fabrication into the wire form commonly used for infiltration brazing.
Elimination of the flux requirement for infiltration brazing is clearly desirable. One attempted solution to the need for a flux in the context of joining brazes has been to add small quantities of lithium to silver-copper alloys to create a xe2x80x9cself-fluxingxe2x80x9d material. These alloys are commercially available from Lucas-Milhaupt, Inc., a division of Handy and Harman Company, under the trade names Lithobraze 720 and Lithobraze 925. Lithium reacts quickly with oxygen to protect the other alloy constituents from oxidation. However, alloys containing reactive elements like lithium are expensive and difficult to prepare, as lithium is readily lost or oxidized during metallurgic processing.
It is therefore an object of the present invention to provide brazing filler metals which do not require a flux.
It is a further object of the present invention to provide fluxless infiltration brazing filler metals which exhibit enhanced infiltration and provide dense, tough composite materials.
It is another object of the present invention to provide infiltration brazing filler metals which are age-hardenable at relatively low temperatures.
It is also an objective of the present invention to provide infiltration brazing filler metals which are simple to manufacture and avoid the presence of manganese, lithium or other extremely reactive elements.
It is an additional object of the present invention to provide infiltration brazing filler metals which impart enhanced lubricity to composite articles.
These and other objects and advantages are provided by the present invention, which is directed to brazing filler metals and, in particular, to brazing filler metals for use in infiltrating porous materials, such as ceramics and porous metals (e.g., metals prepared by powder metallurgy).
The brazing filler metals of the present invention contain two different Group 11 metals and a Group 9 or 10 metal. The first Group 11 metal is typically present in an amount ranging from about 78 to about 99.97% by weight. The second Group 11 metal, which is different from the first metal, is typically present in an amount ranging from about 0.01 to about 12% by weight. The Group 9 or 10 metal is typically present in an amount ranging from about 0.01 to 5% by weight. The brazing filler metal may also contain silicon in an amount ranging from about 0.01 to about 5% by weight. All percentages are based on the total weight of the brazing filler metal. In a more specific embodiment, the invention is directed to brazing filler metals containing silver, copper, nickel, and optionally silicon.
The brazing materials of the present invention do not require a flux, are age-hardenable, exhibit satisfactory infiltration, and impart enhanced lubricity to composite materials and composite articles formed therefrom. The brazing filler metals may be in the form of foil, wires, powders or pastes.
The brazing filler metals of the present invention can be used to infiltrate porous metals, such as powder metallurgy components, and particularly to infiltrate steel powder metallurgy components that are subsequently fabricated into bearings or bushings. Accordingly, the present invention is also directed to composite materials that have a porous matrix phase, typically containing interconnected pores, and a dispersed phase of the brazing filler metal of the present invention, which fills these pores. The present invention is further directed to composite articles fabricated from such composite materials, including bearings, bushings and other load bearing parts.
The present invention is also directed to the process of using the brazing material to make age-hardenable composite articles, by disposing the brazing filler metal adjacent to a surface of the porous matrix material, heating the resulting combination for a time and at a temperature sufficient for the brazing filler metal to wet the porous matrix material and to coat the pores, age-hardening the composite material by heating for a time and at a temperature sufficient to solution-anneal the composite, quenching the solution-annealed material, and heating the quenched component for a time and at a temperature sufficient to cause hardness of the quenched component to increase.
The brazing filler metals of the present invention are capable of infiltrating the pores of the matrix without the need for a flux. Moreover, the brazing filler metals of the present invention exhibit improved infiltration resulting from the elimination of the flux. Avoiding the use of flux prevents clogging of the pores, which can sometimes prevent or complicate infiltration of a matrix component by the brazing filler metal. Thus, the brazing filler metals of the present invention more completely wet and infiltrate porous materials than known brazing filler metals. In addition, the brazing filler metals of the present invention can age-harden at moderate temperatures, avoiding the need for impractical strain-hardening by cold work. The brazing filler metals are easy to cast and do not react with graphite molds as do manganese-containing materials. Further, the brazing filler metals are easily drawn into wire without the pickling process typically required to draw manganese-containing wire. This avoidance of manganese also contributes to the ability of the brazing filler metals to function in the absence of a flux, since the flux is no longer required to limit manganese oxidation.
Composite materials made using the brazing filler metals of the present invention are dense, tough, hard, and have a high lubricity. These materials are very suitable for applications in bearings, bushing or other devices where friction between moving parts must be minimized. For instance, porous steel bearings or bushings impregnated with a brazing filler metal of the present invention have a longer lifetime and exhibit decreased galling relative to bearings made using infiltration brazing metals known in the art.
These objects and advantages, as well as the nature and proper use of the invention, will become apparent from the following description of the invention, and in the accompanying drawings, in which like referenced characters generally refer to the same parts or elements throughout the figures. The detailed description and the drawing figures are not to be interpreted as limiting the invention in any way.